Only important thing is to survive initial invasion by Karl. And, pardon my opinion, that's surprisingly doable now when warscore from battles is capped at 75% for attacker. I saw him divide his army on two stacks which marched around Saxony and took attrition while trying to siege provinces down. Remain operational, abuse tribal armies, run when needed, and immediately retake every holding you can. Attack only when you are absolutely sure of victory. And raid everything not nailed down in time before war.
tl;dr: Even if RNG hates you, you may still win by patience, logistics and ticking warscore. I survived around ten years before he agreed white peace.
Also allies may do more harm than good. They keep losing.